Recently, there has been increasing interest in energy storage technology. Electrochemical devices have been widely used as energy sources in portable phones, camcorders, notebook computers, PCs and electric cars, resulting in intensive research and development. In this regard, electrochemical devices are subjects of great interest. Particularly, development of rechargeable secondary batteries has been the focus of attention. Recently, research and development into an electrode and a battery having a novel design have been conducted in order to improve capacity density and specific energy thereof.
Among the currently used secondary batteries, lithium secondary batteries, developed in early 1990's, have drive voltage and energy density higher than those of conventional batteries using aqueous electrolytes (such as Ni-MH batteries, Ni—Cd batteries and H2SO4—Pb batteries), and thus they are spotlighted in the field of secondary batteries.
The secondary batteries may include a cathode, an anode, a porous separator and a non-aqueous electrolyte. In general, secondary batteries including an anode using a carbonaceous material, and a cathode using a lithium metal oxide have an average discharge voltage of about 3.6 to 3.7V, and a charge/discharge voltage ranging from 0 to 4.2V. However, in the lithium secondary batteries, a safety problem such as fire, explosion, etc. may occur due to the use of a non-aqueous electrolyte, and such a problem may be more significant by increasing the capacity density of the battery.
Especially, continuous charge or overcharge of a non-aqueous electrolyte secondary battery may significantly reduce the safety of the battery, and herein, one of the reasons that may cause such a problem is heat generation by the structural breakdown of a cathode. The operation principle is as follows. A cathode active material, such as a lithium-containing metal oxide capable of absorbing and releasing lithium and/or lithium ions, is deformed into a thermally unstable structure by the release of a large amount of lithium upon overcharge. In a state where the battery has been overcharged, when the battery temperature reaches the critical temperature by external physical impacts, such as high temperature exposure, etc., the structural unstable cathode active material releases oxygen, and an electrolyte solvent causes an exothermic decomposition reaction. The combustion of the electrolyte solution by such an exothermic reaction may be accelerated by oxygen released from the cathode and carried out in a chain-like manner, resulting in the fire and explosion phenomena of the battery, which are cause by thermal runaway. Accordingly, research on the control of such fire or explosion of a battery has been required, and various methods for solving the problems have been provided.
A first method is a method of using a compound subjected to a redox shuttle reaction, such as chloroanisole, as an electrolyte additive for consuming overcharge current. However, this method is disadvantageous in that the effect is not significant at high charge current.
A second method is a method of mechanically blocking current by controlling gas generation upon overcharge. As a representative example, there is introduction of a safety means, such as CID (Current Interrupt Device)-reverse, to a battery. Particularly, when the internal pressure of the battery is increased by gas generation within a cell upon overcharge, the CID-reverse blocks the current flowing through the cell and an additional current flow before fire and rupture occur by thermal runaway. Herein, before the fire and rupture occur by thermal runaway, CID short circuit is required to operate by increase of the internal pressure of the battery.
Accordingly, a material capable of directly increasing the internal pressure of a battery, such as cyclohexylbenzen (which is a compound capable of generating gas through a chemical reaction at overcharge voltage), has been usually added to an electrolyte. However, the use of an alkyl benzene derivative such as cyclohexylbenzen (CHB) may cause a problem in that battery performance is degraded after cycle repetition or continuous charge. Also, such an electrolyte additive is required to be a compound capable of generating a large amount of gas through oxidation at a required overcharge voltage, but an appropriate compound for such a purpose is very restrictive.